This invention relates to surgical structural elements consisting of bioabsorbable or semi-bioabsorbable composites, as well as to products and applications using these materials. Such surgical structural devices may include plates, screws, nails, pegs, rods, pins and any other device which would require a bioabsorbable material with relatively high rigidity and strength.
The use of internal bone fixation is an established and clinically used technique. The two major types of internal fixation devices are bone plates and intramedullary rods. The particular form intended for this invention is as an internal bone fixation plate.
Bone plates are applicable to a large variety of bones including load bearing as well as non-load bearing bones. Presently, more long bone fixations are done with intramedullary rods. Both of these devices are traditionally made of metal, e.g. stainless steel 316. There are two major disadvantages, however, associated with the presently used metal plates:
(1) Metal plates have a modulus approximately one order of magnitude greater than cortical bone. This mismatch in stiffness is known to cause stress protection induced osteoporosis or osteopenia. The increase in porosity and decrease in cortical wall thickness results in a weakened bone which is prone to refracture once the bone plate is removed. PA0 (2) The metal plates must be removed due to their nonbiodegradable nature coupled with the eventual possibility of corrosion. A second surgical procedure is therefore required which may introduce further complications. PA0 (1) The bone plate should provide a firm fixation of the broken bone to promote union during the early stages of healing. PA0 (2) Once union has occurred, the load which was initially supported by the bone plate, would be gradually transferred back to the bone. This would induce the formation of stronger more dense bone at the fracture site thus accelerating the healing process. PA0 (3) After the bone heals (3 to 6 months after implantation) the bone plate would completely lose its ability to support a load. The bone would then be once again subjected to its normal stresses.
The use of lower stiffness materials such as non-absorbable composites and tubular steel have been investigated. However, no human trials of these materials are known.
Lower stiffness bone fixation devices have advantages over metal plates. For example the stiffness of a bone plate can be made essentially equal to the modulus of cortical bone. If the low stiffness bone plate is also bioabsorbable, it does not have to be surgically removed. Thus the need for a second surgical procedure is eliminated.
Considerable research has therefore been devoted to the development of low stiffness bone plate materials. The properties which are most desirable in a bone plate are:
Bioabsorbable materials with an initial modulus and strength at or near those of cortical bone are useful as internal bone fracture fixation devices for load bearing as well as non-load bearing bones.
The completely bioabsorbable or semi-absorbable composites of this invention are superior in mechanical properties and in biological behavior to the stainless steel devices presently used. The mechanical properties of these composites can be tailored to the specific end-use application. The devices of this invention will gradually lose their mechanical properties and will ultimately fully or partially disappear.